Three mesh polarizers are mounted on the entrance wheel, in addition to the free hole normally used in the photometric mode. For both channels they provide polarization analyzers at the 3 position angles 0, +120 and +240 degrees with respect to the Y+ direction. These position angles are the same as for the polarizers in PHOT.
The spectral transmission of the 3 components, for light linearly polarized in their plane of polarization, is very close to the spectral transmission of the filter used in the configuration.
Table 
gives the uncertainty dP on the linear
polarization ratio P, as a function of the signal-to-noise ratio of the
measurement (first column) and of P (first line). dP can also be
computed directly with the following formula:
Table: Accuracy dP on the polarization ratio P, as a function
of P and of the S/N ratio of the measurement
The instrumental polarization will be calibrated in flight. The
pyramid mirror of the telescope, at 45 
incidence angle,
should be the main contributor to it, but the effect is expected to
be low on such a cold gold coated surface.